In such motors, the phenomenon occurs that the boundaries between the individual rotor poles, the so-called “pole boundaries,” seek the positions of the largest air gaps. For the observer, this has the appearance that the pole boundaries are attracted by the slots of the stator. This effect is called “cogging.” The torque created thereby is called “cogging torque” because it seeks to hold the rotor in particular rotational positions, or arrest it as it passes through those positions.
This effect is generated by a so-called “reluctance torque,” i.e. during the rotation of the rotor, relative to the stator, in the magnetic circuit of the motor, in certain rotation angle ranges, magnetic energy is stored, and, in other rotation angle ranges, this magnetic energy is released, analogous to when one alternately tensions a spring and releases it. For the storing, energy must be supplied to the rotor, i.e. the rotor is being braked, and conversely, where the stored energy is being released, the rotor is being driven. If one turns the rotor of such a motor by hand, one has the impression that one “feels every slot.”
In the context of many drive applications, this reluctance torque is disruptive, so that there one is forced to use so-called “ironless” stator windings, in which no reluctance torque arises. However, the power of such motors with ironless stators is generally insufficient because their air gap is very large.